The process of manufacturing semiconductor devices, such as image sensors, typically involves using microlithography to transfer patterns from a set of masks to photosensitive material on a substrate by means of an expose tool. After the photosensitive material is developed, the resulting pattern in the photosensitive material is used as a temporary removable mask for other semiconductor processes. Examples of semiconductor processes include, but are not limited to, etching and implanting. The resulting patterns in the photosensitive material can also be included in a final product. A color filter array or microlens array are examples of some resulting patterns that can be included in an image sensor.
One method for defining patterns in a photosensitive material is known as a step and repeat method. A mechanical surface known as a stage supports a substrate and is configured to accurately move the wafer over given distances. A stepper system is used when circuitry to be fabricated in the substrate is larger than the maximum expose field of the expose tool in the stepper system. The stepper system projects an image onto only a portion of the wafer. Multiple exposures of the pattern are stepped and repeated over the entire wafer. Various exposures could then be “stitched” together to form the required pattern. The terms “stitched” or “stitching” refer to the accurate positioning, or abutting, of one exposure to adjacent exposures.
Prior art stitching approaches typically require a great many expose steps at each patterning level, thereby increasing the amount of time needed to perform the exposure operation. Reducing the number of patterning levels increases the stepper capacity required to efficiently produce semiconductor devices. Moreover, with imaging devices such as image sensors, defects or disruptions in the resulting patterns of the photosensitive material can appear as artifacts in the captured images. In addition to process induced random defects, the disruptions can be caused by seams created as a result of stitching blocks of patterns. Every level of patterning potentially contributes to the production of seam artifacts.